Histocompatibility (H) antigens of the rat will be extracted by exposure of live lymphoid cells to hypertonic salt. After dialysis, the extracts will be fractionated by any of several procedures, including discontinuous gel electrophoresis, anion exchange and molecular sieving chromatography and isoelectric focusing. Fractionation will be monitored by cytotoxin inhibition using rat alloantisera specific for antigens controlled by the major as well as several minor H loci. The antisera will be absorbed to functional monospecificity and the immunogenetic relationships of the antigens they detect will be determined in appropriate parental-hybrid backcross populations. Antigens determined by major and minor loci will be compared for chemical composition and physical characteristics.